The invention relates to a process for the preparation of N,N-dialkylanilines by reacting anilines with lower alcohols or with the corresponding ethers at an increased temperature and increased pressure in the gas phase in the presence of proton-containing zeolites of the pentasil type.
It its known that N,N-dialkylated anilines can be prepared by reacting anilines with alcohols in the presence of acid catalysts, both in the liquid and in the gas phase. In the liquid-phase process, the reaction is carried cut in the presence of a liquid catalyst, such as, for example, sulphuric acid, hydrochloric acid or phosphorus trichloride. These processes must be carried out under pressure. Because of the powerful corroding action of the acid catalysts, it is necessary to carry out the process in autoclaves which are especially resistant to corrosion. Yet, it is not possible to suppress corrosion completely. Moreover, there are substantial complications when the catalyst is removed and disposed of.
DE-AS (German Published Specification) 1,031,796 describes the reaction of aniline with alcohols in a way in which a mixture of aniline and excess alcohol is passed through hot, concentrated phosphoric acid. However, the selectivity for N,N-dialkyl compounds is low, in particular when it is intended to introduce alkyl radicals other than methyl. For example, in the case where ethyl is used as a radical, the temperature is 220.degree. C. and the molar ratio of aniline/ethanol is 1:3, 51% of N-ethylaniline and 15% of N,N-diethylaniline are obtained. According to DE-OS (German Published Specification) 2,658,728, it is possible to improve the yield of N,N-diethylanilin when the process is carried out with the addition of aliphatic amines; conditions which are otherwise analogous give 44% of N-ethylaniline and 36% of N,N-diethylaniline. Apart from the corrosion problems which exist even here, such processes also have the disadvantage that the large amount of phosphoric acid becomes unusable as catalyst after some time and must be replaced by fresh phosphoric acid, with the result that waste disposal problems occur here, too.
U.S. Pat. No. 4,599,449 describes a process for gas-phase alkylation of aromatic amines with alcohols on transition metal oxide catalysts. However, the selectivity to N,N-dialkylanilines is very low. Even when a 5-fold excess of ethanol is used, the reaction rate of the aniline is not higher than 38% and thus very low, and N,N-diethylaniline is only obtained in unsatisfactory yields. Moreover, this process gives a high proportion of by-products which are alkylated on the ring.
DE-AS (German Published Specification) 2,335,906 teaches the N,N-dialkylation of arylamines with alcohols on silica catalysts which are covered with 0.1-20% by weight of phosphoric acid. To obtain good selectivities, however, a very large excess of alcohol of up to 20 mol equivalents is required. On the one hand, this results in a low space-time yield and, on the other hand, the separation and recyclization of the excess alcohol mean complicated distillation procedures. To avoid rapid deactivation of the catalyst and to ensure that its service life is long, it is additionally necessary continuously to add phosphoric acid and/or alkyl phosphates during the alkylation; however, a proportion of these phosphorus compounds is always discharged and contaminates the reaction product, thus requiring complicated separation procedures.
Furthermore, it is known that zeolites can be employed as catalysts for the gas-phase alkylation of aromatic amines with alcohols U.S. Pat. No. 4,801,752 suggest zeolites of the ZSM 5 type having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 20-700. However, flexible control of the yield in the direction of N,N-dialkylaniline is only possible to a limited extent, in particular when the radicals are other than the methyl radicals. The most favourable conditions give a maximum molar selectivity for N,N-diethylaniline of 10.1%. Moreover, temperatures up to above 400.degree. C. must be used in this process. In addition, a significant proportion of unidentified by-products which are alkylated on the ring is obtained.
Japanese Patent 61/35,246 (1986) likewise describes zeolites having a medium pore size as catalysts for the gas-phase alkylation of aromatic amines with alcohols With these catalysts, it is possible to obtain N-monoalkylanilines in high selectivity, the text particularly stressing the fact that the formation of N,N-dialkylanilines on these catalysts is reduced. According to this publication, zeolites having a medium pore size are hence unsuitable for the dialkylation of aromatic amines.
There was therefore still a need to have available a catalytic process with which a large number of variously substituted anilines can be dialkylated on the N atom in the gas phase, this process not having the abovementioned disadvantages; in particular, desired alkyl radicals were those having more than one C atom. It was desired that the catalysts should be distinguished by uncomplicated availability, long service lives and high activities, and they should guarantee substantial reaction rates while having a good selectivity for N,N-dialkylation.